This invention relates to a sensor, and in particular to a force sensor. Force sensors are more important than other types of sensor, as they may be readily adapted to measure other physical properties.
In a known force sensor described in British Patent Application No. GB 2146120A, a light source produces light pulses which are used to induce resonance in a body by means of the photo-acoustic effect in which each of the incident light pulses causes alternate heating and cooling of the body at the frequency of the pulsed light source. The resonance frequency at which the body vibrates depends on a force applied to it. An advantage of this kind of force sensor is that no electrical connection between the vibrating body and the power source is required. The absence of electricity in sensors employing optically driven vibrating bodies is particularly advantageous when the sensors are used in environments where the presence of electricity is hazardous.
A disadvantage of this known type of force sensor is that it is very difficult to lock the optical pulse frequency on to the vibrational resonance frequency when that frequency is continually varying due to changes in the applied force. A separate technique is necessary to monitor the amplitude of vibration of the body or the phase of its vibrational displacement relative to the optical pulses, to provide feedback between the optical pulse frequency and the vibrational resonance frequency. The optical pulse frequency can then be automatically adjusted to follow any change in the resonance frequency, and hence keep the two locked together.